Pressure sensitive adhesives based on low molecular weight epoxidized polydienes are known in the art. These polymers are typically crosslinked using chemical crosslinking agents or by UV or electron beam induced reaction of the epoxy groups to form a high molecular weight network. As taught in European Patent 0 785 974 B1, however, when the polymer is sufficiently epoxidized to yield suitably short reaction times the resulting network is highly crosslinked. As such, adhesives composed of low molecular weight epoxidized polydienes and typical formulating agents such as tackifiers and plasticizing oils, yield strong adhesives which have little pressure sensitive character.
One solution to this problem taught in EP 0 785 974 B1 was the addition of monohydroxylated low molecular weight polydienes. The monohydroxylated polydiene reacted with and thereby consumed some fraction of the epoxy groups without affecting crosslinking. In this approach, adhesive formulations were found such that the crosslinked network was loosened and a proper balance of pressure sensitive adhesive properties.
Another solution to this problem taught in EP 0 785 974 B1 was the replacement of the monohydroxylated polydiene in part or in total by a dihydroxylated polydiene. While formulations were found which gave high gel contents, high temperature holding power was poor.
A need yet exists for UV curable pressure sensitive adhesives which have suitable tack and peel properties but the further improvement of high temperature performance. The adhesive formulations of the present invention address this need. UV curable pressure sensitive adhesive formulations have been found which comprise monohydroxylated epoxidized polydiene polymers, a high molecular weight starblock copolymer, a hydroxylated polydiene having more than one hydroxyl group per molecule, a tackifier and a photoinitiator. Surprisingly, the inclusion of the hydroxylated polydiene having more than one hydroxyl group per molecule did not result in an overly crosslinked adhesive, but rather led to an improved and heretofore unachieved balance of properties.